The present invention relates to integrated circuit memory devices, and in particular to read only memory (ROM) devices having virtual ground and bit lines.
A ROM integrated circuit device includes a ROM storage cell block including storage cells. Bit lines from the storage cells are used to output data from the storage cells during a read operation. Such devices may also include virtual ground lines that may be positioned adjacent the bit lines on the integrated circuit device. As integration density of the device increases, the widths and lengths of lines formed in the integrated circuit device generally are reduced. As a result, electrical coupling may result between adjacent ones of the virtual ground and bit lines.
A variety of approaches may be taken to reduce or prevent such an electrical coupling from adversely affecting the device. For example, when the virtual ground lines and the bit lines are in a precharged state, the supply of the precharge voltage to the bit lines may be stopped when the virtual ground lines are being discharged to ground. As a result, the voltage of the bit lines may be affected by the virtual ground lines and drop to a predetermined level lower than the precharge voltage. To compensate for this drop, the supply of the precharge voltage to the bit lines is not stopped until a predetermined period of time after the virtual ground lines are grounded. Thus, the affect of the virtual ground lines on the bit lines may be reduced or eliminated.
However, because the virtual ground lines and the bit lines are conventionally precharged at the same time, as described above, the precharge voltage is still applied for a predetermined period of time after the virtual ground lines are grounded. Thus, a short-circuit current may flow through the virtual ground lines and, as a result, the virtual ground lines may not be at a ground voltage level. This state may continue until the supply of the precharge voltage to the virtual ground lines stops. If the virtual ground lines are not fully grounded, the operational speed of the ROM integrated circuit device may decrease and its power consumption may increase.
Accordingly, some embodiments of the present invention may provide a ROM semiconductor device that allows virtual ground lines to be fully grounded by preventing short-circuit current from flowing through the virtual ground lines when the virtual ground lines are precharged, discharged and grounded.